Erdogan is ready to abandon a deal to halt he migrant flow into Europe and "open the floodgates" unless Europe gives way, diplomats fear.

For weeks, the E.U. and Ankara have been locked in a high-stakes standoff as Erdogan refused to rewrite sweeping anti-terrorism laws that have been used to prosecute academics and journalists, a precondition of visa-free travel.

It is the key sweetener in the fragile pact of March 18th that has – for now – seen Turkish authorities halt the exodus over the Aegean to Greece. In public, leaders have vowed there would be no deal until all conditions were met.

The visa waiver deal had been due to kick in on July 1st, and there were fears that Erdogan would increase migrant flows if he did not get his way.

In the memo from May 13, the ambassador said Erdogan's pursuit of Jan Boehmermann, a German satirist, has "only strengthened the view that he is an authoritarian bully who is trying to blackmail Europe."

Uwe Corsepius, Mrs Merkel's Europe adviser, told Erdogan was ready to delay, meaning they could avoid a "major escalation of tensions" before Britain's referendum on June 23.

However, a second DipTel, from Richard Moore, the British ambassador to Turkey, is more pessimistic.

It warns: "If visa liberalisation doesn't happen, an impetuous and riled Erdogan – prone to come out fighting when he feels ‘betrayed' – could carry through his threat to ‘open the flood gates' to Europe for migrants."

It floats the idea that Britain could extend the travel rights for around one million Turkish citizens who hold special passports, saying there will be "questions and complaints" about the difficulty of reaching the UK, which is outside the Schengen zone.

"One option would be to assess again the possibility of visa travel for Turkish special passport holders, which would be a risk, but a significant and symbolic gesture to Turkey."

In a joint statement, the Home Secretary and Foreign Secretary said it was "completely untrue" that the "UK is considering granting visa liberalisation to some Turkish citizens."

"The Government's policy is, and will remain, to maintain current visa requirements for all Turkish nationals wishing to visit the UK, regardless of what arrangements other Member States in the Schengen area may make with Turkey."